Mixing two shaders based on raylength
Try increasing the bounces under light paths.



The direct light preset with caustices gives a very dark image 



while full global preset gives a golden brown finish. 



I also think the SSS being used on the backface may be stopping light passing through the object. The calculation for the mix appears to always give a zero value for the visible faces, by replacing the SSS with another glass shader you get a brighter finish.